PROJECT SUMMARY Cellular heterogeneity, the discrete differences between cells within the same population, is recognized as a contributor to malignant conditions. However, the contribution of cellular heterogeneity to normal development is only beginning to be appreciated. The idea that minute differences among cells of the same type are capable of determining cell state is especially pertinent when considering the development of the germline. Functional heterogeneity among primordial germ cells (PGCs, the embryonic precursors to sperm and egg cells) is evident from their inception; as PGCs migrate individually from their site of specification to the gonadal ridges (Ginsburg et al., 1990; Kunwar et al., 2006), they spread into early migratory Leaders and later migratory Laggards (Gomperts et al., 1994). Additionally, not every PGC will make it to the gonadal ridge; many PGCs die along the migratory route. Successful colonizers of the gonadal ridge maintain the potential to contribute gametes to the next generation. Ex vivo cell culture experiments suggest that independent of tissue niche, PGCs exhibit signaling responses to migratory cues that vary at the single cell level (Cant et al., 2016). These differences in migratory potential suggest that there may be subtypes within the population of PGCs that are intrinsically determined, but how these discrete cell-to-cell differences impact development is unknown. Distinctions between Leaders and Laggards may be heritable through mitosis, transcriptionally distinct, and reflect differences in metabolic function. These preliminary data fuel my overall hypothesis that PGC migration acts selectively to favor more fit PGCs while excluding those that are less fit. I expect that loss of stable, heritable epigenetic states, inability undergo metabolic processes, or cell damage may render PGCs less fit, manifesting in altered RNA expression at the transcriptional level. This proposal aims to characterize how heterogeneities among germline cells impact development and reproductive capacity, and how migration is a mechanism to ensure only the highest quality cells are present for the next generation. How PGCs are selected to generate the next generation is not known. Understanding the basis of PGC heterogeneity and selection could reveal how development limits the transmission of gametes and shapes inheritance. Therefore, I suggest that PGCs that successfully colonize the nascent gonad do not do so by random chance, but rather through a migration-based selection to ensure the most fit cells contribute to future species lineage.